Relationship between Ventilation/Perfusion Ratio Mismatch and Capnogram Phase III Slope in Acute Bronchoconstriction and Its Modulation by Salbutamol

Q: What does the phase III slope (SIII) represent in this study?

SIII is evaluated as a noninvasive marker of ventilation/perfusion (V˙A/Q˙) mismatch. The study examines how SIII relates to ventilation distribution and perfused blood volume distribution during acute bronchoconstriction.

Q: How were ventilation (V˙A) and perfusion (VB) measured?

The researchers used synchrotron radiation K-edge subtraction imaging computed tomography to quantify V˙A distribution and VB as a surrogate of perfusion in mechanically ventilated rabbits.

Q: Why might gas exchange not improve after salbutamol despite mechanical improvement?

After salbutamol, low V˙A/Q˙ regions received increased perfusion and the shunt fraction increased, leading to unchanged or paradoxically worsened gas exchange.

Sep 30, 2026

Uploaded by dhado

Phase III slope (SIII) of the volumetric capnogram is proposed as a noninvasive marker for ventilation/perfusion (V˙A/Q˙) mismatch, yet direct links to measured ventilation and perfusion patterns remain limited, especially during acute bronchoconstriction. Using a rabbit methacholine model, synchrotron K-edge subtraction imaging quantified lung ventilation distribution (V˙A) and perfused blood volume (VB) before and after salbutamol. SIII related significantly to V˙A and most strongly to mean V˙A/VB, with regression showing a larger ventilation contribution than perfusion. Salbutamol improved mechanics but increased shunt and worsened gas exchange.

LanguageEnglish

CategoryHealthcare

FormatPDF

Pages12

Reading Time30 min

AccessFree to download

What We already Know about This Topic

Volumetric capnography phases

Ventilation vs perfusion contribution

What This article Tells Us That Is New

Synchrotron radiation imaging approach

Abstract

Background

Methods

Results

Conclusions

CritiCal Care MediCine | Relationship between Ventilation/Perfusion Ratio Mismatch and Capnogram Phase III Slope in Acute Bronchoconstriction and Its Modulation by Salbutamol Assessed by Synchrotron Imaging in Rabbit Lung
Sam Bayat, M. D. , Ph. D. , Gergely Albu, M. D. , Ph. D.
| | | --- | --- | | Gergely H. Fodor, M. D. , Ph. D. ,
Mathieu Guilbart, M. D. , Liisa Porra, Ph. D. , Ferenc Petak, Ph. D. , D.Sc. ,
Barna Babik, M. D. , Ph. D Anesthesiology 2026; 144:419–30 | | editOr’S PerSPeCtiVe What We already Know about This Topic • Volumetric capnography of the expiratory carbon dioxide waveform is widely used for patient monitoring in the operating room and intensive care unit • Consisting of three separate phases, it reflects emptying of the conducting airways (phase I), a mixture of gas from the alveoli and the conducting airways (phase II), and finally the alveolar gas (phase III) • Phase III is postulated to reflect not only ventilation inhomogeneity but also perfusion inhomogeneity • The relative contribution of these has been difficult to study What This article Tells us That Is New • Synchrotron radiation imaging allows quantitative evaluation of the distribution of lung ventilation, blood volume, and tissue morphology at a subacinar spatial resolution aBStraCt Background: The slope of phase III (SIII) of the volumetric capnogram is proposed as a noninvasive marker of ventilation/perfusion ratio (V˙A /Q˙) mismatch. However, direct comparisons of SIII with measurements of lung ventilation and perfusion distribution are lacking, particularly in acute bronchoconstriction. This study was developed to assess the relationship between SIII and distribution of lung ventilation (V˙A) and perfused blood volume (VB), a surrogate of perfusion, using synchrotron radiation K-edge subtraction imaging in a rabbit model of methacholine-induced acute bronchoconstriction. Methods: Five anesthetized, mechanically ventilated rabbits underwent imaging of V˙A and VB using K-edge subtraction imaging computed tomography before and after methacholine-induced bronchoconstriction and subsequent salbutamol administration. Mean V˙A , VB, V˙A/VB, and their heterogeneity were quantified. SIII was measured from exhaled volumetric capnograms. Multiple linear regression was used to evaluate the relative contributions of V˙A and VB to SIII. results: Methacholine-induced ventilation defects decreased both V˙A (mean difference [d] = 0.22, 95% CI, 0.12 to 0.51; P < 0.001) and VB (d = 1.9 ⋅ 10−3; 95% CI, 1.0 × 10−3 to 2.8 × 10−3; P< 0 .001) and increased their heterogeneity (d = 0 .42; 95% CI, 0.17 to 0 .67; P< 0 .001; and d = 0 . 12; 95% CI, 0.05 to 0 . 19; P < 0 .001, respectively) . Salbutamol thereafter improved respiratory elastance (d = 726.2; CI, 130.9 to 1,321; P = 0 .04) but decreased V˙A/VB (d = 47 .3; 95% CI, 26.5 to 68 . 1; P = 0.028), leading to increased shunt fraction (d = 0.044; 95% CI, 0.039 to 0 .049; P < 0 .001) . SIII was significantly related with V˙A (r = 0 .49; 95% CI, 0.12 to 0.739; P < 0.001) but not with VB. The strongest association was observed between SIII and mean V˙A/VB (r = −0 .65; P < 0 .0001) . Multiple regression demonstrated that V˙A had a larger effect on SIII than VB. Conclusions: SIII is determined not only by V˙A heterogeneity but also by V˙A /Q˙ and its heterogeneity. The lack of improvement or paradoxical worsening in gas exchange despite mechanical improvement after salbutamolin acute bronchoconstriction is due to increased perfusion of low V˙A /Q˙ regions and shunt fraction. These findings provide new insight into monitoring mechanical ventilation and acute bronchoconstriction with capnography.(Anesthesiology 2026; 144:419–30) • Using methacholine to induce bronchospasm (causing ventilation defects) followed by salbutamol in this rabbit model, the authors demonstrated that phase III is determined not only by ventilation heterogeneity but also by ventilation/perfusion heterogene

FAQs

What does the phase III slope (SIII) represent in this study?

SIII is evaluated as a noninvasive marker of ventilation/perfusion (V˙A/Q˙) mismatch. The study examines how SIII relates to ventilation distribution and perfused blood volume distribution during acute bronchoconstriction.

How were ventilation (V˙A) and perfusion (VB) measured?

The researchers used synchrotron radiation K-edge subtraction imaging computed tomography to quantify V˙A distribution and VB as a surrogate of perfusion in mechanically ventilated rabbits.

Why might gas exchange not improve after salbutamol despite mechanical improvement?

After salbutamol, low V˙A/Q˙ regions received increased perfusion and the shunt fraction increased, leading to unchanged or paradoxically worsened gas exchange.

Sep 30, 2026

Uploaded by dhado